Method of fabricating heavily doped region in double-diffused source MOSFET (LDMOS) transistor

ABSTRACT

A transistor includes a source, a drain and a gate. The source includes a p-doped p-body, a p+ region overlapping the p-body, an n+ region overlapping the p-body in proximity to the p+ region, and an n-doped source, heavily double-diffused (SHDD) region, only into the source region of the transistor, the SHDD region having a depth about equal to that of the first n+ region and overlapping the first n+ region. The drain includes a second n+ region and an n-doped shallow drain overlapping the second n+ region. The gate includes a gate oxide and a conductive material over the gate oxide. The SHDD region extends further laterally than the first n+ region beneath the gate oxide. The SHDD region is implanted using a dopant concentration greater than that of the n-doped shallow drain but less than that of the first n+ region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/344,167, filed Dec. 24, 2008, which claims priority to U.S.Application Ser. No. 61/017,530, filed on Dec. 28, 2007, the entiredisclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to semiconductor devices.

BACKGROUND

Voltage regulators, such as DC to DC converters, are used to providestable voltage sources for electronic systems. Switching voltageregulators (or simply “switching regulators”) are known to be anefficient type of DC to DC converter. A switching regulator generates anoutput voltage by converting an input DC voltage into a high frequencyvoltage, and filtering the high frequency input voltage to generate theoutput DC voltage. Specifically, the switching regulator includes aswitch for alternately coupling and decoupling an input DC voltagesource, such as a battery, to a load, such as an integrated circuit. Anoutput filter, typically including an inductor and a capacitor, iscoupled between the input voltage source and the load to filter theoutput of the switch and thus provide the output DC voltage. Acontroller, such as a pulse width modulator or a pulse frequencymodulator, controls the switch to maintain a substantially constantoutput DC voltage.

LDMOS transistors are commonly used in switching regulators as a resultof their performance in terms of a tradeoff between their specificon-resistance (R_(dson)) and drain-to-source breakdown voltage (BV_(d)_(—) _(s)). Conventional LDMOS transistors are typically fabricatedhaving optimized device performance characteristics through a complexprocess, such as a Bipolar-CMOS (BiCMOS) process or a Bipolar-CMOS-DMOS(BCD) process, that includes one or more process steps that are notcompatible with sub-micron CMOS processes typically used by foundriesspecializing in production of large volumes of digital CMOS devices(e.g, 0.5 μm DRAM production technologies), as described in greaterdetail below. As a result, conventional LDMOS transistors are,therefore, not typically fabricated at such foundries.

A typical sub-micron CMOS process used by foundries specializing inproduction of large volumes of digital and mixed signal CMOS devices,referred to herein as sub-micron CMOS process, will now be described. Asub-micron CMOS process is generally used to fabricate sub-micron CMOStransistors—i.e., PMOS transistors and/or NMOS transistors having achannel length that is less than 1 μm. FIG. 1 shows a PMOS transistor100 and an NMOS transistor 102 fabricated through a sub-micron CMOSprocess on a p-type substrate 104. The PMOS transistor 100 isimplemented in a CMOS n-well 106. The PMOS transistor 100 includes asource region 108 and a drain region 110 having p-doped p+ regions 112and 114, respectively. The PMOS transistor 100 further includes a gate116 formed of a gate oxide 118 and a polysilicon layer 120. The NMOStransistor 102 is implemented in a CMOS p-well 122. The NMOS transistor102 includes a source region 124 and a drain region 126 having n-dopedn+ regions 128 and 130, respectively. The NMOS transistor 102 furtherincludes a gate 132 formed of a gate oxide 134 and a polysilicon layer136.

FIG. 2 illustrates a sub-micron CMOS process 200 that can be used tofabricate large volumes of sub-micron CMOS transistors (such as the CMOStransistors shown in FIG. 1). The process 200 begins with forming asubstrate (step 202). The substrate can be a p-type substrate or ann-type substrate. Referring to FIG. 1, the CMOS transistors arefabricated on a p-type substrate 104. A CMOS n-well 106 for the PMOStransistor and a CMOS p-well 122 for the NMOS transistor are implantedinto the substrate (step 204). The gate oxide 118, 134 of each CMOStransistor is formed, and a CMOS channel adjustment implant to controlthreshold voltages of each CMOS transistor is performed (step 206). Apolysilicon layer 120, 136 is deposited over the gate oxide 118, 134,respectively (step 208). The p+ regions of the PMOS transistor and then+ regions of the NMOS transistor are implanted (step 210). The p+regions 112, 114 and n+ regions 128, 130 are highly doped, and providelow-resistivity ohmic contacts. In a sub-micron CMOS process, formationof an n+ region typically occurs through a three-step process in asingle masking and photolithography step as follows: 1) a lightly dopedn-type impurity region is implanted, 2) an oxide spacer is formed, and3) a heavily doped n+ impurity region is implanted. Formation of a p+region occurs in a similar manner. The formation such n+ and p+ regionsallow transistors to have an improved hot carrier performance.

Foundries specializing in production of large volumes of digital CMOSdevices generally have fixed parameters associated with the foundries'sub-micron CMOS process. These fixed parameters are typically optimizedfor the mass production of digital sub-micron CMOS transistors. Forexample, in process step 206, the CMOS channel adjustment implantgenerally has an associated thermal budget that is typically fixed, andhas parameters optimized for mass production of sub-micron CMOStransistors.

As discussed above, conventional LDMOS transistors typically achieveoptimized device performance through a complex process, such as a BiCMOSprocess or a BCD process, that includes one or more process steps thatare not compatible with a sub-micron CMOS process optimized for the massproduction of digital sub-micron CMOS transistors.

FIG. 3A shows a conventional LDMOS transistor 300 fabricated through aBiCMOS process on a p-type substrate 302. The LDMOS transistor 300includes source region 304 with an n-doped n+ region 306, a p-doped p+region 308, and a p-doped p-body diffusion (p-body) 310. The LDMOStransistor 300 also includes a drain region 312 with an n-doped n+region 314 and an n-type well (HV n-well) 316, and a gate 318, includinga gate oxide 320 and a polysilicon layer 322.

In the BiCMOS process, the gate oxide 320, and gate oxide of any CMOStransistors fabricated in the BiCMOS process, is formed prior toimplantation of the n+ region 306 and the p-body 310. The BiCMOSprocess, therefore, allows the gate 318 to serve as a mask duringimplantation of the n+ region 306 and the p-body 310—i.e., the n+ region306 and the p-body 310 are self aligned with respect to the gate 318.The self aligned lateral double diffusion of the n+ region 306 and thep-body 310 forms the channel of the LDMOS transistor 300.

Such kinds of self aligned double diffusions are not easily integratedinto a sub-micron CMOS process because the subsequent drive-in step (orthermal budget) associated with self aligned double diffusions disruptsthe fixed thermal budget associated with sub-micron CMOS process steps(e.g., process step 206) and requires a redesign of the thermal budgetallocated to the sub-micron CMOS process steps. That is, the selfaligned double diffusions generally includes a drive-in step with a longduration and a high temperature that can cause the characteristics ofsub-micron CMOS transistors (e.g., threshold voltages) to shift.

The lateral doping profile in region (a) of the LDMOS transistor 300controls the tradeoff between the on-resistance R_(dson) and thedrain-to-source breakdown voltage BV_(d) _(—) _(s). The vertical dopingprofile in region (b) determines the drain-to-substrate breakdownvoltage BV_(d) _(—) _(sub) of the LDMOS transistor, and the pinch-offdoping profile in region (c) determines the source-to-substratepunch-through breakdown voltage BV_(s) _(—) _(sub) of the LDMOStransistor. The source-to-substrate punch-through breakdown voltageBV_(s) _(—) _(sub) is an important parameter for an LDMOS transistorwith a floating operation requirement, e.g, an LDMOS transistorimplemented as a high-side control switch in a synchronous buck circuitconfiguration.

FIG. 3B shows a conventional LDMOS transistor 330 fabricated through aBCD process on a p-type substrate 332. The LDMOS transistor 330 includessource region 334 with an n-doped n+ region 336, a p-doped p+ region338, and a p-doped p-body 340. The LDMOS transistor 330 also includes adrain region 342 with an n-doped n+ region 344 and an n-type layer (HVn-Epi) 346, and a gate 348, including a gate oxide 350 and a polysiliconlayer 352. As with the BiCMOS process, in the BCD process, the gateoxide 350, and gate oxide of any CMOS transistors fabricated in the BCDprocess, is formed prior to implantation of the n+ region 336 and thep-body 340.

In the BCD process, an n+ buried layer 354 can be grown on the p-typesubstrate 332 to improve the source-to-substrate punch-through breakdowncharacteristics of the LDMOS transistor. Such an approach offers animproved tradeoff between the on-resistance R_(dson) and drain-to-sourcebreakdown voltage BV_(d) _(—) _(s) of the LDMOS transistor as thelateral doping profile of the LDMOS transistor can be optimized withoutconstrain on the vertical doping profiles. However, such a BCD processincludes the growth of the HV n-Epi layer 346, and this step isgenerally not compatible with a sub-micron CMOS process.

Another approach used in a BCD process is to utilize an n− layer 360implanted in the drain region 362 of the LDMOS transistor 364 as shownin FIG. 3C. The n− layer 360, n+ region 366, and p-body 368 are selfaligned with respect to the gate 370—i.e., the n− layer 360, n+ region366, and p-body 368 are implanted after formation of gate oxide 372. Theinclusion of the n− layer 360 provides an additional parameter tofurther optimize the tradeoff between the on-resistance R_(dson) anddrain-to-source breakdown voltage BV_(d) _(—) _(s) of the LDMOStransistor. Similar to the n+ buried layer approach of FIG. 3B, theinclusion of the n− layer 360 at the surface provides a method todecouple vertical and horizontal doping constraints.

SUMMARY

In one aspect, a method of fabricating a transistor having a sourceregion, a drain region, and a gate region on a substrate, includesimplanting, into a surface of the substrate, a n-doped n-well, forming agate oxide between a source region and a drain region of the transistor,covering the gate oxide with a conductive material, implanting, into thesource region of the transistor, a p-doped p-body, implanting, into thesource region of the transistor, a first n-doped n+ region to overlapthe p-body, implanting, only into the source region of the transistor, asource, heavily double-diffused (SHDD) region to overlap the p-body, theSHDD region being an n-doped region implanted to a depth about equal tothat of the first n-doped n+ region, the SHDD region extending furtherlaterally than the first n-doped n+ region beneath the gate oxide, aportion of the SHDD region overlapping a portion of the first n-doped n+region, implanting, into the source region of the transistor, a p-dopedp+ region, the p-doped p+ region in proximity to the first n-doped n+region, implanting, into the drain region of the transistor, a secondn-doped n+ region, and implanting into the drain region an n-dopedshallow drain, wherein the SHDD region is implanted using a dopantconcentration greater than that used in the implant of the n-dopedshallow drain but less than that used in the implant of the firstn-doped n+ region.

Implementations can include one or more of the following. The SHDDregion can encompass the entire first n-doped n+ region. An oxide spacermay be formed on each side of the gate oxide after implanting the SHDDbut before implanting the first n-doped n+ region and the second n-dopedn+ region. The oxide spacer may be formed prior to formation of thefirst n-doped n+ region and the second n-doped n+ region. The SHDD maybe formed after formation of the gate oxide. In the source region, asurface area of the SHDD region, a surface area of the first n-doped n+region, and a surface area of the p-doped p+ region may be locatedwithin a surface area of the p-doped p-body. The p-doped p+ region mayabuts the first n-doped n+ region.

In another aspect, a transistor includes a source, a drain and a gate tocontrol a depletion region between the source and the drain. The sourceincludes a p-doped p-body, a p-doped p+ region overlapping the p-body, afirst n-doped n+ region overlapping the p-body in proximity to thep-doped p+ region, and a n-doped source, heavily double-diffused (SHDD)region, only into the source region of the transistor, the SHDD regionhaving a depth about equal to that of the first n-doped n+ region andoverlapping the first n-doped n+ region. The drain includes a secondn-doped n+ region and an n-doped shallow drain overlapping the secondn-doped n+ region. The gate includes a gate oxide and a conductivematerial over the gate oxide. The SHDD region extends further laterallythan the first n-doped n+ region beneath the gate oxide. The SHDD regionis implanted using a dopant concentration greater than that used in theimplant of the n-doped shallow drain but less than that used in theimplant of the first n-doped n+ region.

Implementations can include one or more of the following. The second n+region may extend deeper than the n-doped shallow drain. The SHDD regionmay encompass the entire first n-doped n+ region. The p-doped p+ regionmay abut the first n-doped n+ region.

In another aspect, a method of fabricating a transistor having a sourceregion, a drain region and a gate region on a substrate, includesforming a gate oxide between a source region and a drain region of thetransistor, covering the gate oxide with a conductive material,implanting, into the source region of the transistor, a p-doped p-body,implanting, into the source region of the transistor, a first n-doped n+region overlapping the p-body, implanting, only into the source regionof the transistor, a source, heavily double-diffused (SHDD) region tooverlap the p-body, a portion of the SHDD region overlapping a portionof the first n-doped n+ region, the SHDD region extending furtherlaterally than the first n-doped n+ region beneath the gate oxide,implanting, into the source region of the transistor, a p-doped p+region overlapping the p-body, the p-doped p+ region in proximity to thefirst n-doped n+ region, implanting, into the drain region of thetransistor, a second n-doped n+ region, implanting into the drain regionan n-doped shallow drain, the n-doped shallow drain extending beneaththe gate oxide to contact the p-doped p-body, and implanting, into thesource region of the transistor, a p-doped p+ region.

Implementations can include one or more of the following. The SHDDregion may encompass the entire first n-doped n+ region. The SHDD regionmay be implanted to a depth about equal to that of the first n-doped n+region. The SHDD region may be implanted using an impurity concentrationlower than that used in implanting of the first n-doped n+ region. TheSHDD region may be implanted using a level of dopant greater than thatused in the implant of the n-doped shallow drain. The p-doped p+ regionmay abut the first n-doped n+ region.

In another aspect, a transistor includes a source, a drain and a gate tocontrol a depletion region between the source and the drain. The sourceincludes a p-doped p-body, a p-doped p+ region overlapping the p-body, afirst n-doped n+ region overlapping the p-body in proximity to thep-doped p+ region, and a n-doped source, heavily double-diffused (SHDD)region only in the source region of the transistor, the SHDD regionoverlapping the p-body, a portion of the SHDD region overlapping thefirst n-doped n+ region. The drain includes a second n-doped n+ regionand an n-doped shallow drain. The gate includes a gate oxide and aconductive material over the gate oxide. The SHDD region extendingfurther laterally than the first n-doped n+ region beneath the gateoxide. The n-doped shallow drain extends beneath the gate oxide tocontact the p-doped p-body.

Implementations can include one or more of the following. The second n+region may extend deeper than the n-doped shallow drain. The secondn-doped n+ region may be self-aligned to the gate of the transistor. Thefirst n+ region may be surrounded by the p-body. The p-body may bedeeper than the p+ region, the first n+ region and the SHDD region. TheSHDD region may encompass the entire first n-doped n+ region. The SHDDregion may be implanted to a depth about equal to that of the firstn-doped n+ region. The SHDD region may be implanted using a impurityconcentration less than that used in the implant of the first n-doped n+region. An outer boundary of the SHDD region may be aligned with anouter boundary of the first n-doped n+ region. The p-doped p+ region mayabut the first n-doped n+ region.

In another aspect, a voltage regulator has an input terminal to becoupled to an input voltage source, an output terminal to be coupled toa load, a power switch including a transistor of an aspect above, and afilter to provide a generally DC output voltage at the output terminal.A duty cycle of the power switch controlling power supplied to theoutput terminal.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a conventional PMOStransistor and NMOS transistor formed on a p-type substrate.

FIG. 2 is a flow diagram illustrating a conventional sub-micron CMOSprocess for manufacturing CMOS transistors.

FIGS. 3A, 3B, and 3C are schematic cross-sectional views of conventionalLDMOS transistors.

FIGS. 4A-4B are schematic cross-sectional views of an SHDD LDMOStransistor, and a three-dimensional view of the surface area of thesource and drain regions of the SHDD LDMOS transistor, respectively.

FIG. 4C is a schematic cross sectional views of another implementationof a SHDD LDMOS transistor.

FIG. 5 is a flow diagram illustrating an exemplary process formanufacturing a semiconductor transistor, including an SHDD LDMOStransistor, that is compatible with a sub-micron CMOS process.

FIGS. 6A-6L illustrate the exemplary processes of manufacturing an SHDDLDMOS transistor, a PMOS transistor, and an NMOS transistor, accordingto the exemplary processes of FIG. 6.

FIGS. 7A-7B are schematic cross-sectional views of an SHDD LDMOStransistor with an extended N-LD region, and a three-dimensional view ofthe surface area of the source and drain regions of the same,respectively.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 4A shows a schematic cross-sectional view of certainimplementations of the LDMOS transistor 416. The LDMOS transistor 416can be a power switch, e.g., a power switch in a voltage regulator,e.g., a low-side power switch connecting an intermediate terminal toground in a buck converter topology, a high-side power switch connectingan input voltage to ground in a buck converter topology, or a powerswitch in boost converter topology, buck-boost converter topology, orvarious transformer-coupled topologies. In such a voltage regulator, aduty cycle of the power switch controls power supplied to the outputterminal, and filter provides a generally DC output voltage at theoutput terminal.

The LDMOS transistor 416 can be fabricated on a n-type well (HV n-well)500B, such as a high-voltage n-type well, implanted in a p-typesubstrate 502. An HV n-well implant is typically a deep implant and isgenerally more lightly doped relative to a CMOS n-well. The HV n-well500B can have a retrograded vertical doping profile. The LDMOStransistor 416 generally includes a source region 506, a drain region508, and a gate 507.

Referring again to FIG. 4A, the source region 506 generally includes ap-doped p+ region 515, an n-doped n+ region 517, and a p-doped p-body522. The drain region 508 generally includes an n-doped n+ region 525and an n-doped shallow drain (N-LD) 527. The n+ region 525 can beshallower than the N-LD 527 (as shown in FIG. 4A), or the n+ region 525can extend deeper than the N-LD 527 (as shown in FIG. 4C). The sourceregion 506 further includes an n-type source, heavily double-diffused(SHDD) region 518 (in some contexts, the SHDD region can be consideredto be part of the n+ region). The SHDD region 518 of the LDMOStransistor can be implanted using similar techniques performed withrespect to N-LDD regions in conventional CMOS processes.

As shown, the SHDD region 518 overlaps a portion of the n-doped n+region, and can extend under the gate oxide 512 further than the n+region 517. In these implementations, the SHDD can be implanted beforethe formation of the oxide spacer, thus permitting the SHDD region 518to extend further into the channel than the n+ region 517.

If the N-LDD regions the CMOS transistors on the substrate use similardoping depth and concentration, then the SHDD region 518 can beimplanted simultaneously and with the same process as the N-LDD regionsin any CMOS transistors on the substrate. Separate masks can be employedfor implanting the SHDD region 518 and the n-doped n+ region 517, thuspermitting placing the SHDD selectively on the source regions.Alternatively, the SHDD region 518 and the n-doped n+ region 517 can beimplanted using a same mask to control, for example, the overlappingregion between the SHDD region 518 and the n-doped n+ region 517. Inthese implementations, the n-doped n+ region 525 in the drain would beimplanted using a mask different from that used in forming the n-dopedn+ region 517 in the source so that no SHDD is implanted in the drain.Using different masks also can provide flexibility with respect to, forexample, the relative dopant concentration of the n doped n+ region 525and the n-doped n+ region 517.

In some implementations, the SHDD region 518 can extend to about thesame depth as the n-doped n+ region 517 (i.e., SHDD region 518 canextend to about the same depth into the substrate 502 as does then-doped n+ region 517). The boundary of the SHDD region 518 farther fromthe gate can be located closer to the gate than the outer boundary ofn-doped n+ region 517, or it can be aligned with a boundary of then-doped n+ region 517 and abut the boundary of the p-doped p+ region515.

The HV n-well 500B, the N-LD 527, and the n+ region 525 in the drainregion 508 are volumes containing doped material. Likewise, the n+region 517, the p+ region 515, and the p-body 522 in the source region506 are volumes containing doped material. In some implementations, boththe N-LD 527 and the HV n-well 500B can have a lower concentration ofimpurities than that of the n+ regions 517 and 525. Portions at whichthese volumes overlap may have a higher doping concentration than theindividual volumes separately. For example, a portion 524 that containsthe overlapping volumes of the n+ region 525, the N-LD 527, and the HVn-well 500A can have the highest doping concentration among otheroverlapping volume portions. A portion 526 that contains the overlappingvolumes of the N-LD 527 and the HV n-well 500B excluding the n+ region525, can have a lower doping concentration than that of the portion 524.A portion 504 that only includes the HV n-well 500B can have a lowerdoping concentration than that of the portion 524 or 526, because itdoes not include multiple overlapping doped volumes.

The SHDD region 518 may be implanted using a lower doping concentrationimplantation step than that used for the n+ region 517, and a higherdoping concentration implantation step than that used for N-LD region526. Thus, in some implementations, the portions in which the SHDDregion 518 and the n+ region 517 overlap can have a higher dopingconcentration of impurities than the non-overlapping portions. In theseimplementations, the volume containing the SHDD region 518 (i.e., SHDDportion 520) can also have a doping concentration higher than that ofthe p-body 522.

Referring to FIG. 4B, volumes of the p+ region 514, n+ regions 516 and524, SHDD region 520, p-body 522 and N-LD region 526 can each havesurface areas on the surface 532 of the LDMOS transistor 416. The HVn-well 500B has a surface area 534. For example, in the drain region508, the portion 526 of the N-LD region has a surface area 536 locatedwithin the surface area of the HV n-well 500B. The portion 524 of the n+region has a surface area 538 located within the surface area 536 of theportion 526 of the N-LD region. In the source region 506, the p-body 522has a surface area 540 located within the surface area 534. The portion514 of the p+ region and the portion 516 of the n+ region have a surfacearea 544 and 542, respectively, each being located within the surfacearea 540 of the p-body 522.

In implementations where a SHDD region 518 is diffused into the p-body522, the portion 520 of the SHDD region 518 also can have a surface area548 located within the surface area 534. The portion of the SHDD region518 overlapping the portion 516 of the n+ region can have a surface area546 on the LDMOS transistor 416.

FIG. 5 illustrates an exemplary process 600 of fabricating asemiconductor device, including an SHDD LDMOS transistor, a PMOStransistor with floating operation capability (i.e., the source of thetransistor is not grounded), and an NMOS transistor with floatingoperation capability, that is compatible with a sub-micron CMOS process.

Process 600 begins with forming a substrate (step 602). The substratecan be a p-type substrate or an n-type substrate. Referring to theexample of FIG. 6A, a semiconductor layer including a p-type substrate502 is formed. Next, as shown in FIG. 6B, HV n-wells 500A for the LDMOStransistor, and 500B for the PMOS transistor with floating operationcapability and NMOS transistor with floating operation capability, areimplanted into the p-type substrate 502 (step 604). In someimplementations, the HV n-wells 500A and 500B can be integrated as asingle well. Alternatively, the HV n-wells 500A and 500B can beimplanted as separate wells. The HV n-wells 500A and 500B also can beimplanted simultaneously or sequentially.

A CMOS n-well 106 (for example, for a PMOS transistor), and a CMOSp-well 122 (for example, for a NMOS transistor), are implanted into thep-type substrate 502, as illustrated in FIG. 6C (step 606). While it isillustrated that the CMOS n-well 106 and the CMOS p-well 122 are formedafter the HV n-wells 500A and 500B, the order can be reversed so thatthe CMOS n-well 106 and the CMOS p-well 122 are formed prior toimplanting the HV n-wells 500A and 500B. In some implementations, the HVn-wells 500A and 500B, and the CMOS n-well 106, can be implantedsimultaneously, for example, by using a single mask. In otherimplementations, each of the HV n-wells 500A and 500B, and the CMOSn-well 106, can be implanted sequentially (and in any order).

Referring to FIG. 6D, a p-body for the NMOS transistor with floatingoperation capability can be implanted (step 608). For example, a p-body700 for the NMOS transistor with floating operation capability can beimplanted into the HV n-well 500A.

After implantation of the p-body 700 for the floating NMOS transistor,the gate oxide for each of the LDMOS transistor, the PMOS transistorwith floating operation capability, the NMOS transistor with floatingoperation capability, and the CMOS transistors, can be formed (step610). In some implementations, each gate oxide can be simultaneously orsequentially formed. For instance, the gate oxide for the LDMOStransistor can be formed at the same time as the gate oxide of the CMOStransistors so that the LDMOS transistor may establish a similarthreshold voltage and gate oxide thickness as those of the CMOStransistors. Alternatively, the gate oxide of the LDMOS transistor canbe formed at a different time or with a different thickness than thegate oxide of the CMOS transistors to flexibly allow the LDMOStransistor to be implemented with a dedicated gate oxide thicknesslarger or smaller than that of the CMOS transistors. In theseimplementations, when the gate oxide of the LDMOS transistor is formedto be thicker than the gate oxide of the CMOS transistors, the LDMOStransistor can allow higher gate drive-in applications where a lowervoltage power supply may not be readily available. This flexibilityyields optimization of the LDMOS transistor depending on specificrequirements of a power delivery application, such as efficiency targetsat a particular frequency of operation.

The gate oxide for the LDMOS 512 can be formed on the surface 702 of thep-type substrate 502 above the HV n-well 500B (step 610). Similarly, thegate oxide 706A of the PMOS transistor (with floating operationcapability) and the gate oxide 706B of the NMOS transistor (withfloating operation capability) can be formed on the surface of thep-type substrate 502 above the HV n-well 500A. Further, the gate oxide118 and the gate oxide 134 can be formed on the surface of the p-typesubstrate 502 above the CMOS n-well 106, and on the surface of thep-type substrate 502 above the CMOS p-well 122, respectively.

As shown in FIG. 6E, a polysilicon layer is then deposited over the gateoxide (step 612). The polysilicon layer can be used as transistorelectrodes for interconnection purposes. As shown in FIG. 6E,polysilicon layers 510, 708A and 708B can be deposited over the gateoxide 512, the gate oxide 706A, and the gate oxide 706B, respectively.Also, a polysilicon layer 120 and a polysilicon layer 136 are depositedover the gate oxide 118 formed above the CMOS n-well 106 and the gateoxide 134 formed above the CMOS p-well 122, respectively.

As illustrated in FIG. 6F, in certain implementations, a self-alignedp-body 522 for the source region of the LDMOS transistor can then beimplanted (step 614). The p-body 522 is implanted into the HV n-well500B in these implementations.

Referring to FIG. 6G, a shallow drain (N-LD) 527 is implanted anddiffused into the drain of the LDMOS transistor (step 618). In someimplementations, the shallow drain 527 can be implanted before or afterthe LDMOS gate is formed (i.e., the shallow drain 527 can be non-selfaligned or self aligned with respect to the gate 507 of the LDMOStransistor). The shallow drain 527 can be implanted using the LATimplant or a normal angle tilt implant as discussed above.

At step 620, implantation for the SHDD region 518 is performed, followedby the implantation for the n+ regions at step 624. Once the SHDD region518 is formed, as shown in FIG. 6H, the LDMOS transistor is implantedwith an n+ region 525 at the drain and an n+ region 517 at the source.The n+ regions 710 and 712 are implanted at the drain and source,respectively, of the NMOS transistor with floating operation capability.The n+ regions 128 and 130 also are implanted at the source and drainregions, respectively, of the CMOS p-well 122.

While it is shown that the SHDD region 518 is implanted prior toimplantation for the n+ regions, depending on a desired alignment of theSHDD with respect to the gate, the order can be reversed so that theSHDD regions 518 is implanted after implantation of the n+ regions. Ineither implementations, the SHDD region 518 can be about the same depthas the n+ region 517 (i.e., the SHDD region 518 extends toward thep-type substrate 502 to about the same depth as the n+ region 517).

After the SHDD region 518 and the n+ regions are formed, p+ regions ofthe LDMOS transistor, the PMOS transistor with floating operationcapability, the NMOS transistor with floating operation capability, andthe CMOS transistors, are implanted (step 626). As shown in FIG. 6I, thep+ regions 714A and 714B are implanted at the drain and source,respectively, of the PMOS transistor with floating operation capability.A p+ region 515 is also implanted at the source of the LDMOS transistor.Separate p+ regions 112, 114, are implanted at the source and drain,respectively, of the PMOS transistor. Each of the p+ regions can beformed separately or simultaneously.

FIGS. 6J-6L show the process of step 616 in more detail. Referring toFIG. 6J, after the shallow drain (N-LD) 527 is implanted and diffusedinto the drain of the LDMOS transistor (e.g., step 614), the SHDD region518 is implanted into the source of the LDMOS transistor (step 620). TheSHDD region 518 can be implanted to extend under the gate oxide 512previously formed on the HV n-well. The SHDD region 518 may align withan outer boundary (e.g., the boundary away from the drain) of the n+region 517, and abut the p+ region 515. Alternatively, the SHDD region518 may be implanted with a predetermined distance away from the p+region 515.

Then, as shown in FIG. 6K, a pair of oxide spacers 530 can be formedadjacent to the gate oxide 512 and the polysilicon 510 (step 622). Afterthe oxide spacers are formed, implantation for the n+ regions isperformed (step 624). The LDMOS transistor can be implanted with an n+region at the drain and another n+ region at the source. The n+ region517 and 525 can be formed over the SHDD region 518 and the N-LD region527, respectively, as illustrated in FIG. 6L. The n+ regions also can beimplanted at the drain and source of the NMOS transistor with floatingoperation capability, and at the source and drain regions of the CMOSp-well 122. Depending on a design application, the n+ regions can beperformed prior to formation of the oxide spacers.

The p+ region can be formed by a two-step implant in a manner similar tothe n+ region. That is, a SHDD region can be implanted before formationof the oxide spacer, and a p+ region can be implanted after formation ofthe oxide spacer.

Because the gate may have some finite source/drain overlap, in theseimplementations, the gate (or gate oxide) can be formed first and thenused as a diffusion or implant mask in defining the source and drainregions so as to preclude the source and/or drain from running under thegate oxide. Once the gate is formed, the gate can serve as a mask duringimplantation of the n+ regions and the p-body so that they areself-aligned with respect to the gate. As shown, n+ regions 517 and 525of the LDMOS transistors are implanted and self-aligned with respect tothe corresponding gate oxide.

In some implementations, only one side (e.g., the source) of the LDMOStransistor includes an SHDD region. For example, the n+ region 525 canbe formed using a one-step process because the drain of the LDMOStransistor does not include a SHDD region.

In some implementations, steps 602-626 may be performed in the orderlisted, in parallel (e.g., by the same or a different process,substantially or otherwise non-sequentially), or in a different order toachieve the same result. For example, after forming the p-type substrate502, CMOS n-well 106 and CMOS p-well 122 can be implanted prior toimplanting the HV n-wells 500A and 500B. As another example, the p+regions can be formed prior to implanting the n+ regions, and the SHDDregion can be formed prior to implanting the N-LD region. As yet anotherexample, the N-LD region 527 can be implanted prior to forming the gateoxide or implanting the self-aligned p-body.

However, the order discussed above is not limited to that shown. Forexample, the n+ region 517 can be implanted prior to forming the SHDDregion 518, such that the SHDD region 518 self-aligns with the gateoxide 512 and overlaps the n+ region 517 after the SHDD region 518 issubsequently formed.

In some implementations, steps 602-626 may be performed in the orderlisted, in parallel (e.g., by the same or a different process,substantially or otherwise non-sequentially), or in different order toachieve the same result. For example, n+/p+ regions can be implantedprior to forming the oxide spacers. As another example, the SHDD regioncan be implanted prior to implanting the N-LD region. As yet anotherexample, the SHDD region can be formed prior to or subsequently afterany one of the steps 618, 622, 624 and 626.

In other implementations, depending on a particular design application,one or more of the steps 602-626, or combinations thereof can bebypassed. In yet other implementations, any of the steps 602-626 may beperformed by two or more processes rather than by a single process,performed simultaneously or sequentially.

The processes 600 provides a potential advantage over conventionaltechniques because any channel length variation due to misalignment ofthe p-body 522 and n+ region 516 can be mitigated and compensated by agreater critical dimension (CD) control of the process 600.

Also, PMOS transistors are typically formed on a CMOS n-well. Inapplications where a shift in threshold voltages of CMOS transistors istolerable, a PMOS transistor can be directly implemented in an HVn-well, such as the PMOS transistor with floating operation capabilityin the example of FIG. 6H. Implementing a PMOS transistor directly in anHV n-well has the advantage of allowing the process 600 to skip a CMOSn-well implant and masking step (while maintaining its thermal cycle),thereby potentially lowering the overall process manufacturing cost.

The SHDD with the combination of an implant depth about equal to the n+region 516 and a dopant concentration greater than that used in theimplant of the n-doped shallow drain but less than that used in theimplant of the n+ region 516 provides a good R_(dson) while increasingthe safe operating area (SOA) of the transistor, i.e., the ability ofthe transistor to sustain high current with high drain voltage.

FIGS. 7A-7B illustrate yet other implementations of the LDMOS transistor416. In these implementations, both SHDD region 518 is present (seeFIGS. 4A-4B), and N-LD region 826 is extended under gate 507 (see FIGS.7A-7B) to contact the p-body. In certain implementations, N-LD region826 may also overlap with SHDD region 518 and/or n-doped n+ region 516.FIG. 7B illustrates the corresponding surface areas and volumes of theseimplementations. This configuration can provide a similar R_(dson) inlight of lower gate drive voltage.

The SHDD can be incorporated into transistor structures in a similarfashion as the N-LDS region described in U.S. Patent Publication No.2007-0207600, the disclosure of which is incorporated by reference. Anumber of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the LDMOS transistor is not limited to DC-DC converterapplications; the transistor may be used in applications such as LEDdrivers and RF amplifiers. Accordingly, other implementations are withinthe scope of the following claims.

What is claimed is:
 1. A method of fabricating a transistor having asource region, a drain region, and a gate region on a substrate, themethod comprising: implanting, into a surface of the substrate, an-doped n-well; forming a gate oxide between the source region and thedrain region of the transistor; covering the gate oxide with aconductive material; implanting, into the source region of thetransistor, a p-doped p-body; implanting, into the source region of thetransistor, a first n-doped n+ region to overlap the p-body, implanting,only into the source region of the transistor, a source, heavilydouble-diffused (SHDD) region to overlap the p-body, the SHDD regionbeing an n-doped region implanted to a depth about equal to that of thefirst n-doped n+ region, the SHDD region extending further laterallythan the first n-doped n+ region beneath the gate oxide, a portion ofthe SHDD region overlapping a portion of the first n-doped n+ region,the first n-doped n+ region extending further laterally than the SHDDregion toward the source region; implanting, into the source region ofthe transistor, a p-doped p+ region, the p-doped p+ region in proximityto the first n-doped n+ region; implanting, into the drain region of thetransistor, a second n-doped n+ region; and implanting into the drainregion an n-doped shallow drain, wherein the SHDD region is implantedusing a dopant concentration greater than that used in the implant ofthe n-doped shallow drain but less than that used in the implant of thefirst n-doped n+ region.
 2. The method of claim 1, further comprisingforming an oxide spacer on each side of the gate oxide after implantingthe SHDD region but before implanting the first n-doped n+ region andthe second n-doped n+ region.
 3. The method of claim 2, where the oxidespacer is formed prior to formation of the first n-doped n+ region andthe second n-doped n+ region.
 4. The method of claim 2, where the SHDDregion is formed after formation of the gate oxide.
 5. The method ofclaim 1, where, in the source region, a surface area of the SHDD region,a surface area of the first n-doped n+ region, and a surface area of thep-doped p+ region are located within a surface area of the p-dopedp-body.
 6. The method of claim 1, wherein the p-doped p+ region abutsthe first n-doped n+ region.
 7. A method of fabricating a transistorhaving a source region, a drain region and a gate region on a substrate,the method comprising: forming a gate oxide between the source regionand the drain region of the transistor; covering the gate oxide with aconductive material; implanting, into the source region of thetransistor, a p-doped p-body; implanting, into the source region of thetransistor, a first n-doped n+ region in the p-body; implanting, onlyinto the source region of the transistor, a source, heavilydouble-diffused (SHDD) region to overlap the p-body, a portion of theSHDD region overlapping a portion of the first n-doped n+ region, theSHDD region extending further laterally than the first n-doped n+ regionbeneath the gate oxide, the first n-doped n+ region extending furtherlaterally than the SHDD region toward the source region; implanting,into the source region of the transistor, a p-doped p+ region in thep-body, the p-doped p+ region in proximity to the first n-doped n+region; implanting, into the drain region of the transistor, a secondn-doped n+ region; and implanting into the drain region an n-dopedshallow drain, the n-doped shallow drain extending beneath the gateoxide to contact the p-doped p-body; and implanting, into the sourceregion of the transistor, a p-doped p+ region.
 8. The method of claim 7,wherein the SHDD region is implanted to a depth about equal to that ofthe first n-doped n+ region.
 9. The method of claim 7, wherein the SHDDregion is implanted using an impurity concentration lower than that usedin implanting of the first n-doped n+ region.
 10. The method of claim 7,wherein the SHDD region is implanted using a level of dopant greaterthan that used in the implant of the n-doped shallow drain.
 11. Themethod of claim 7, wherein the p-doped p+ region abuts the first n-dopedn+ region.
 12. The method of claim 7, wherein the second n+ region isimplanted deeper than the n-doped shallow drain.
 13. The method of claim7, wherein the first n+ region is implanted to be surrounded by thep-body.
 14. The method of claim 7, wherein the p-body is implanteddeeper than the p+ region, the first n+ region and the SHDD region.